What is the correct action if most imams delay the transitional takbir, such as "Sami'a Allahu liman hamidah" (Allah hears whoever praises Him), until after reaching the rukn (bowing) or achieving tranquility in it, which causes confusion and makes the worshipper doubt whether they have caught the rak'ah (unit of prayer), and does the imam's prayer become void according to Ahmad if he delays the transitional takbir?

A rak'ah can be caught if the worshiper joins the Imam in bowing (ruku') before the Imam raises his head from it. The bowing is considered sufficient if the worshiper bends to the extent that he could place his hands on his knees, even if he does not actually place them there. As for the transitional takbirat, the majority opinion is that they are sunnah (recommended) and not obligatory. The prescribed way is for them to coincide with the transition, so if the worshiper begins them before the transition or completes them after it, his prayer is valid, and praying behind him is permissible.